There is known a spark plug in which a ground electrode has a tip welded to an electrode base thereof for improvement in spark wear resistance. Japanese Laid-Open Patent Publication No. H11-121143 discloses a technique for inspecting this type of ground electrode, including the steps of: capturing image data about a surface of the electrode base to which the tip has been welded; detecting respective edges (i.e. boundaries of contrast changes) of the electrode base and the tip by analysis of the image data; and judging whether the welding position of the tip is proper (acceptable) or not based on the detected edge positions.